This invention relates to once-through steam generators and, in particular, to a furnace wall tubing arrangement located at the upper end of a spirally wound furnace.
In once-through steam generators the mass flow through the furnace wall tubes varies in proportion to the load on the boiler. At the minimum through flow condition there must be sufficient velocity in the tubes to prevent their overheating, because of poor heat transfer between the water and the tube. Since insufficient velocity may be obtained with vertical tubes lining the walls, one method used for increasing the velocity is to form a spiral wound furnace where the tubes wind around the furnace walls in the form of a helix. The number of tubes required to cover the furnace wall is substantially reduced and accordingly increased velocity occurs in each tube.
At the upper end of the furnace, provision must be made for the outlet of the flue gases. Continuing the helix in this area would create substantial complications in tubing layout and accordingly vertical tubes are resorted to in this area. Since the heat absorption rate is less than that down in the furnace area, a lower mass flow in each tube can be tolerated, and accordingly one method of cooling the upper walls involves trifurcating each of the spiral wound tubes into three tubes which pass vertically upward, thereby covering the upper furnace walls.
When operating at subcritical conditions each of these triforcates tends to operate as a steam water separator with the plurality of the water continuing to the farther most tube while the closer tubes tend to carry more steam. At high loads the maldistribution is no problem since all three tubes still remain at the same temperature and sufficient pressure drop is available to pass the flow upwardly through all the tubes.
At low loads, however, this water separating phenomena causes the farther most tube to fill partially with water. All the additional flow is thereafter passing up the two closer tubes until a sufficient load is reached such that the pressure drop through these tubes will push the water leg through the farther most tube. Once the solid water leg has been broken through, the flow resumes in all three circuits.
While this is occurring, the upper portion of the blocked tube receives no cooling and accordingly is susceptable to overheating. There is potential damage due to overheating itself and also potential damage caused by water quenching the hot tube once the flow starts.
It appears that it would be possible to continue the single tube upwardly into a vertically sinuous loop making three passes on the wall instead of trifurcating to the three separate curcuits. This, however, creates problems in potential steam or air trapping in the upper return bend and water pocketing in the lower return bend. The air pocket could be a problem during acid washing of the unit where insufficient flow is achieved to create a pressure differential sufficient to clear that particular tube. Accordingly, that entire circuit would be blocked and not washed.
The lower return bend being blocked by water creates an undrainable circuit which precludes completely draining the unit after an acid wash.